The presence of low levels of sulfur compounds in hydrocarbon and other chemical streams causes serious problems in the refining of petroleum products, the production of plastics, and other chemical processes. Even at parts per billion concentrations, sulfur compounds can poison the expensive catalysts used in refining and chemical processing. As a result of these and other problems with sulfur contamination, a wide variety of analytical techniques have been developed to identify and quantify trace levels of sulfur-containing compounds in different sample matrices.
Current and pending regulations limit the total sulfur content in fuels and other hydrocarbon products. For example, proposed new standards for gasoline and diesel fuels are 40 mg S/kg and 500 mg S/kg respectively. The desired specifications for total sulfur content of polymer grade ethylene and propylene are 50 ppb (0.05 mg S/kg). These new specifications will require improved methods for the determination of total sulfur content in these hydrocarbons which provide higher sensitivity and selectivity than existing methods.
Techniques for the measurement of total sulfur content include x-ray fluorescence, conversion of sulfur compounds to hydrogen sulfide followed by radiometric colorimetry detection, and combustion of sulfur compounds to form sulfur dioxide with detection by UV fluorescence and other techniques. The detection limits for these techniques are typically in the high parts per million range and cannot be used for the measurement of sulfur at low parts per million or parts per billion concentrations.
In some cases, it is desired to measure the concentrations of individual sulfur-containing compounds. This requires the use of a chromatographic technique to separate the sulfur species for subsequent detection, usually using a sulfur-selective detector. In other cases, only the total concentration of sulfur species is required, which eliminates the need for chromatographic separation.
Representative of the prior art for sulfur-selective chromatographic detectors is the flame photometric detector (FPD) described in U.S. Pat. No. 3,489,489 by Brody and Chaney for Flame Photometric Detector with Improved Specificity to Sulfur and Phosphorus. Sulfur compounds are combusted in a hydrogen-rich/air flame to produce electronically excited diatomic sulfur (S.sub.2 *). The emission of radiation from this species is monitored using a photomultiplier tube (PMT) and an optical filter. Since the emitting species is short-lived, the PMT and filter are positioned in close proximity to the flame. The FPD has a non-linear and compound-dependent response for sulfur compounds. The response of the detector is dramatically decreased or quenched when sulfur compounds coelute from the chromatographic system at the same time as larger amounts of non-sulfur species, particularly hydrocarbons.
An improved FPD was described by Patterson and employs a dual flame design. The first hydrogen/air flame is used to partially combust the sample and minimize perturbation in the temperature of the second hydrogen/air flame, where the emitting species is formed upon additional reactions. This design served to reduce the quenching of the sulfur response due to coelution of hydrocarbons, however, quenching is not completely eliminated and this FPD design also has a compound-dependent, non-linear response for sulfur compounds.
Despite these limitation of the FPD, several systems for the measurement of total sulfur content have been developed using the FPD. A method and apparatus for the determination of carbon and sulfur content of hydrocarbon samples was described by Szakasits and Krc using a combination of gas chromatography, and oxidizing furnace to convert hydrocarbon to CO.sub.2 and sulfur compounds to SO.sub.2 and subsequent detection of CO.sub.2 by infrared absorption and detection of SO.sub.2 using an FPD. A total sulfur analyzer based on gas chromatographic separation of sulfur compounds, catalytic hydrogenation of sulfur compounds in a ceramic pipe reactor to form H.sub.2 S and detection of H.sub.2 S using an FPD is described in U.S. Pat. No. 5,049,508 by Hilscher, et al. for Apparatus and Process for Total Sulfur Determination. Hydrocarbons in the sample are hydrogenated to produce low molecular weight compounds. The pipe reactor is constructed from high purity alumina (&gt;99.8% pure) and operated at elevated temperatures, preferably 1150.degree. C..+-.2.degree. C. The detection limit of this analyzer is reported to be 0.3 mg S/kg.
In both of these total sulfur analyzer, an FPD is used for the detection of the sulfur species and therefore these analyzers suffer from the limitation of the FPD. Specifically, there is a non-linear response and potential quenching of the sulfur response due to the presence of non-sulfur species.
A sulfur detection system that employs ozone-induces chemiluminescence has been described in U.S. Pat. Nos. 4,352,779 and 4,678,756, both by Parks. The sample is first passed through a furnace containing a heated metal oxide catalyst to oxidize the sulfur compounds and the other species in the sample matrix. The resultant gas stream is then dried and enters a second furnace where the stream is mixed with hydrogen to reduce the sulfur compounds to form hydrogen sulfide. The gas stream is then dried a second time and enters a chemiluminescent reaction chamber where it is mixed with ozone. Hydrogen sulfide will undergo a multi-step reaction with ozone to produce sulfur dioxide in an electrically excited state which emits light in the blue and ultraviolet region of the spectrum: EQU H.sub.2 S+3O.sub.3 .fwdarw..fwdarw..fwdarw.SO.sub.2 *+H.sub.2 O+3O.sub.2 SO.sub.2 *.fwdarw.SO.sub.2 +hv
The major advantage of the chemiluminescent detection system of Parks over previous FPD-based systems is that a linear response for sulfur compounds is obtained. However, there are serious limitations to the detection scheme disclosed by Parks. To overcome interference from the sample matrix, a furnace containing a metal oxide catalyst, such as granular copper/copper oxide is employed. As described by Parks, the catalyst must be regenerated, either by stopping the analyzer and passing oxygen gas over the catalyst bed, or continuously feeding oxygen to the catalyst bed. This is because, as previously noted, sulfur compounds are known to poison catalysts, by reducing their effectiveness and eventually requiring replacement or regeneration of the catalysts.
Another disadvantage of the Parks process is the requirement for drying the gas stream, not once, but twice. Drying is required after the oxidation catalyst bed, presumably to improve the reaction efficiency of the hydrogenation reaction. Drying is also required after the hydrogenation reaction. Hydrogen sulfide, the product of the hydrogenation reaction will react with water yielding products that will not chemiluminescence with ozone, thus requiring the second drying procedure.
Another serious limitation of the Parks process and also of the Hilscher, et al. process, is the reactivity of H.sub.2 S. Adsorption and loss of hydrogen sulfide on tubing and other components of the analyzers is well known, particularly at low parts per billion levels. This limits the sensitivity of these analyzers to the high parts per billion or even parts per million levels of sulfur.
Finally, the chemiluminescent reaction of H.sub.2 S with O.sub.3 described in the Parks' patents is a multi-step reaction. Since the emitting species is SO.sub.2 *, a large excess of O.sub.3 is required to ensure formation of the emitting species and operating conditions of the analyzer must be adjusted to permit the reactions to occur in front of the photomultiplier tube.
An improved sulfur chemiluminescence detection system was adapted for chromatography detection in U.S. application Ser. Nos. 07/759,105 for Apparatus for Simultaneous Measurement of Sulfur and Non-Sulfur Containing Compounds and 07/754,889 for Process for Simultaneous Measurement of Sulfur and Non-Sulfur Containing Compounds, both by Godec, et al., and assigned to the assignee of the present invention. In the SULFUR CHEMILUMINESCENCE DETECTOR (SCD) brand detector disclosed therein, sulfur monoxide is produced from the combustion of sulfur containing compounds in a hydrogen-rich/air flame. The flame gases are then collected and transferred to a reaction with ozone in a single step to produce electronically excited sulfur dioxide, which emits light in the blue and ultraviolet regions of the spectrum: EQU SO+O.sub.3 .fwdarw.SO.sub.2 *+O.sub.2 SO.sub.2 *.fwdarw.SO.sub.2 +hv
This process has been used for atmospheric monitoring, and has also been combined with a conventional flame ionization detector to provide simultaneous detection of hydrocarbon and sulfur compounds in chromatographic analyses.
An improved burner for the production of sulfur monoxide and subsequent detection of sulfur monoxide by ozone-induced chemiluminescence has been recently described in application Ser. No. 07/824,852 filed Jan. 23, 1992 for Method and Apparatus for the Measurement of Sulfur Compounds by Shearer and assigned to the assignee of the present invention. A ceramic burner is used in place of the hydrogen/air flame to provide increased sensitivity and ease of operation for the measurement of sulfur-containing compounds in chromatographic analyses.
Common to these sulfur chemiluminescent detection systems are several important advantages over existing methods. The response of these systems for sulfur compounds is linear over at least four orders of magnitude. The response is not compound-dependent, but rather an equimolar response is obtained for all sulfur compounds, and there is no quenching of the sulfur response due to co-elution of hydrocarbons, provided the levels of hydrocarbons do not exceed several milligram of carbon per second. No intermediate oxidation, hydrogenation or drying steps are required. Sulfur monoxide is also less reactive than hydrogen sulfide and low parts per billion levels of sulfur compounds can be easily measured, indicating that adsorption and loss of SO is not as much of a problem as with H.sub.2 S.
The major disadvantages of these sulfur chemiluminescent detection systems is the selectivity versus hydrocarbons. While selectivites of &gt;10.sup.6 S/C are obtained when the SCD is used in conjunction with chromatography, higher selectivites are required for non-chromatography analyses and continuous, on-line monitoring. In the previous SCD's when a large sample of hydrocarbon is analyzed, the hydrocarbon is not completely combusted in the flame or ceramic burner resulting in a detector response due to chemiluminescent reactions of ozone with the unburned or partially burned hydrocarbon. Using a chromatographic column to produce a broad hydrocarbon peak decreases but does not eliminate this interference by reducing the amount of hydrocarbon reaching the flame or burner at any given time; however, the use of a chromatographic column is not desired for use in a total sulfur analyzer.
Other art in the field includes U.S. Pat. Nos. 4,843,016 by Fine; 4,717,675 by Sievers, et al; 4,077,774 by Neti; 4,097,230 by Patterson; 3,880,587 by Szakasits; and ASTM Standard D2622, Annual Book of ASTM Standards, Vol. 05.02, ASTM Philadelphia, Pa. 1990; ASTM Standard D1552, Annual Book of ASTM Standards, Vol. 05.01, ASTM Philadelphia, Pa.